1. Field of the Invention
The present invention relates in general to the phase control of a video cassette recorder (referred to hereinafter as VCR), and more particularly to a phase control apparatus for a VCR in which a control signal reproduced by a control head is amplified at a varied amplification gain and a phase of a capstan motor is accurately detected on the basis of the amplified control signal.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional phase control apparatus for a VCR. As shown in this drawing, the conventional phase control apparatus comprises a control head 20 for reproducing a control signal recorded on a control track under a video track of a tape 10, and a control signal amplification circuit 30 for amplifying the reproduced control signal from the control head 20 at a predetermined amplification gain. The control signal amplification circuit 30 is provided with resistors R1 and R2, condensers C1-C3 and an amplifier OP1.
A control signal wave-shaping circuit 40 is provided in the phase control apparatus to wave-shape an output signal from the control signal amplification circuit 30 and stabilize the wave-shaped signal. The control signal wave-shaping circuit 40 includes a level limiter 41 for wave-shaping the output signal from the control signal amplification circuit 30 and a Schmidt trigger circuit 42 for stabilizing an output signal from the level limiter 41.
A phase control circuit 50 is also provided in the phase control apparatus to detect an phase error and a rotation speed of a capstan motor CM in response to an output signal from the control signal wave-shaping circuit 40 and an output frequency from the capstan motor CM and output phase and speed detect signals in accordance with the detected results to allow a video head to accurately trace the video track of the tape 10. The phase control circuit 50 includes a reference pulse generator 51 for generating a reference pulse signal, a phase detector 52 for comparing a phase of the output signal from the control signal wave-shaping circuit 40 and that of the reference pulse signal from the reference pulse generator 51. detecting the phase error of the capstan motor CM in accordance with the compared result and outputting the phase detect signal in accordance with the detected result, and a speed detector 53 for detecting the rotation speed of the capstan motor CM in response to the output frequency from the capstan motor CM and outputting the speed detect signal in accordance with the detected result.
A capstan motor driving circuit 60 is also provided in the phase control apparatus to drive the capstan motor CM in response to the phase and speed detect signals from the phase control circuit 50. The capstan motor driving circuit 60 is provided with a resistor R3, condensers C4 and C5 and an amplifier OP2.
The operation of the conventional phase control apparatus with the above-mentioned construction will hereinafter be described.
First, the control si-gnal recorded on the control track of the tape 10 is reproduced by the control head 20 and then applied to the control signal amplification circuit 30. In the control signal amplification circuit 30, the amplifier OP1 amplifies the reproduced control signal from the control head 20 at the amplification gain determined by the resistors R1 and R2 and the condensers C1-C3. The output signal from the control signal amplification circuit 30 is applied to the control signal wave-shaping circuit 40.
In the control signal wave-shaping circuit 40, the output signal from the control signal amplification circuit 30 is wave-shaped by the level limiter 41 for removal of a noise component therefrom and then stabilized by the Schmidt trigger circuit 42. The output signal from the Schmidt trigger circuit 42 is applied to the phase control circuit 50.
In the phase control circuit 50, the phase of the output signal from the Schmidt trigger circuit 42 is compared with that of the reference pulse signal from the reference pulse generator 51 by the phase detector 52. As a result of the comparison, the phase detector 52 detects the phase error of the capstan motor CM and outputs the phase detect signal. Also, the output frequency from the capstan motor CM is applied to the speed detector 53, thereby causing the rotation speed of the capstan motor CM to be detected by the speed detector 53. As a result of the detection, the speed detector 53 outputs the speed detect signal.
The phase detect signal from the phase detector 52 is applied to a non-inverting input terminal (+) of the amplifier OP2 in the capstan motor driving circuit 60 and the speed detect signal from the speed detector 53 is applied to an inverting input terminal (-) thereof. The amplifier OP2 amplifies the phase detect signal from the phase detector 52 and the speed detect signal from the speed detector 53 at an amplification gain determined by the resistor R3 and the condensers C4 and C5. Then, an output signal from the amplifier OP2 is applied as a drive signal to the capstan motor CM. As a result, the phase error of the capstan motor CM is corrected by the drive signal from the, capstan motor driving circuit 60 so that the video head can accurately trace the video track of the tape 10.
The above-mentioned operation of the conventional phase control apparatus will hereinafter be described in detail with reference to FIGS. 2 to 5. FIG. 2 is a waveform diagram of the output signal from the control signal amplification circuit 30, FIG. 3 is a waveform diagram of the output signal from the control signal wave-shaping circuit 40, FIG. 4 is a waveform diagram of the output signal from the reference pulse generator 51 in the phase control circuit 50 and FIG. 5 is a waveform diagram of the output signal from the phase detector 52 in the phase control circuit 50.
First, the control signal recorded on the control track of the tape 10 is reproduced by the control head 20 and then amplified by the control signal amplification circuit 30, as shown in FIG. 2. The output signal from the control signal amplification circuit 30 as shown in FIG. 2 is wave-shaped and stabilized by the control signal wave-shaping circuit 40, as shown in FIG. 3. The output signal from the control signal wave-shaping circuit 40 as shown in FIG. 3 is applied to the phase control circuit 50.
In the phase control circuit 50, the phase of the output signal from the control signal wave-shaping circuit 40 as shown in FIG. 3 is compared with that of the reference pulse signal as shown in FIG. 4. The phase detect signal as shown in FIG. 5 is generated as a result of the comparison. When the video head traces accurately the video track, the phase detect signal as shown in FIG. 5 is generated in the middle of a rising edge of the reference pulse signal as shown in FIG. 4. But, in the case where the video head traces inaccurately the video track, the phase detect signal is generated before or after the middle of the rising edge of the reference pulse signal as shown in FIG. 4. This signifies the generation of the phase error of the capstan motor CM. As a result, the capstan motor drive signal is generated to correct the phase error of the capstan motor CM.
However, in the case where the tape is bad in quality or the video head travels missing the video track of the tape, the reproduced control signal is low in level, thereby making the phase control difficult. Also, when a noise component is contained in the control head, it is reproduced and amplified together with the control signal, resulting in the difficulty in the phase control. These may result in a faulty operation of the VCR.